1. Field of the Invention
The present invention relates to a film bulk acoustic-wave resonator (FBAR) using a longitudinal vibration mode along a thickness direction of a piezoelectric layer, a filter implemented by a plurality of FBARs, and a method for manufacturing the FBAR.
2. Description of the Related Art
As recent wireless communication technology makes rapid developments and new architectures in wireless communication systems are employed one after another, there is a greater demand for communication devices that are compatible with a plurality of different communication systems. In addition, because the move to higher performance and higher function of mobile terminals increases to an extraordinary number of components, further miniaturization and module configuration of the components are required more and more. And since a filter circuit especially occupies a large volume among various passive components used in a wireless communication circuit, filters must be miniaturized and be merged into a module, so that the wireless communication circuit may be miniaturized and the number of the components may be decreased.
LC filters, dielectric filters and surface acoustic wave (SAW) filters are used in mobile communication devices of earlier technology. The LC filter and the dielectric filter, however, are difficult to be miniaturized and thinned. Especially a coil (inductor) used in the LC filter is difficult to be miniaturized. Further, since resistance losses must be considered in the coil (inductor), a LC filter having a sharp filter characteristic, cannot be realized. Therefore, the SAW filter is used as RF filters and IF filters in mobile communication devices. However, since the resonance frequency of the SAW device is inversely proportional to a space between comb-type electrodes, in a frequency band exceeding one GHz, the space between comb-type electrodes becomes 1 micrometer or less. Therefore, it is difficult to meet the demand of higher frequency of operation, as the frequency increases higher and higher recently. And such configuration using the SAW device needs fundamentally specific components, for instance, special substrates such as lithium tantalate (LiTaO3), which is not suitable for miniaturization.
For a substitute of the SAW device, a resonator which has attracted attention recently is a FBAR using a longitudinal vibration mode along a thickness direction of a piezoelectric layer. The FBAR is called “bulk acoustic wave (BAW)” device, etc. A representative structure of the FBAR of earlier technology is disclosed in Japanese Published Unexamined Patent Application No. 2000-69594. As shown in the Japanese Published Unexamined Patent Application No. 2000-69594, the FBAR is manufactured by the following formation process sequences. First, a cavity is formed on a Si substrate by anisotropic etching, and next, a sacrificial layer, which is easily etched by a specific etchant, for instance, boro-phosphate-silicateglass (BPSG), is buried in the cavity at the top surface of the substrate. Afterwards, the sacrificial layer is polished flat until a top surface of the Si substrate comes into view. By the process, the sacrificial layer is embedded in the cavity at the top surface of the Si substrate and the top surface of the Si substrate surrounds the perimeter of the sacrificial layer while the top surface of the Si substrate being bare. On the sacrificial layer, a bottom electrode, a piezoelectric layer and a top electrode are deposited successively. Afterwards, an etchant supplying conduit is excavated until the etchant supplying conduit reaches the top surface of the sacrificial layer. Then, the sacrificial layer is removed by selective etching by the etchant supplied through the etchant supplying conduit, and a cavity is selectively formed under the bottom electrode. By such a sequence of formation processes, the FBAR is completed. Because, the filter implemented by a plurality of FBARs utilizes a longitudinal bulk vibration along a thickness direction of a piezoelectric layer of the FBAR, a resonator operating at higher frequency bands can be manufactured with ease, by thinning the film thickness of the piezoelectric layer. And since the fabrication accuracy of the piezoelectric layer along a planar direction is enough around one micrometer level, an increase in the manufacturing cost ascribable to the move to higher frequency operations is not caused. Also, it is not necessary for the FBAR to be fabricated on a piezoelectric substrate such as the SAW filter, the FBAR is comparatively easy to fabricate on a silicon (Si) substrate and a gallium arsenide (GaAs) substrate, and the filter implemented by the plurality of FBARs can be monolithically integrated in a single LSI chip. Because, in the FBAR, the resonance frequency is determined by acoustic wave velocity and film thickness of the piezoelectric layer, the resonance frequency is two GHz at a film thickness of one to two micrometers and is five GHz at a film thickness of 0.4 to 0.8 micrometer. Therefore, the FBAR facilitates higher frequency operations up to several decades GHz.
In earlier technology, materials having low elastic losses (internal friction) such as molybdenum (Mo), tungsten (W), iridium Ir), are regarded to be advantageous as materials of the FBAR (see U.S. Pat. No. 5,587 620).
According to the manufacturing method of the FBAR disclosed in Japanese Published Unexamined Patent Application No. 2000-69594, the sacrificial layer is formed at the top surface of the Si substrate, and the bottom electrode is formed on the sacrificial layer, and further, on the top surface of the bottom electrode, a piezoelectric thin film is stacked. Then, the piezoelectric thin film is delineated so as to form a pattern of a piezoelectric layer, occupying a limited area, and the top electrode is delineated on the piezoelectric thin film. In the sequence of formation processes, after the piezoelectric thin film is delineated so as to form the pattern of the piezoelectric layer, a pattern of an extraction wiring of the bottom electrode is required to be delineated by wet etching using such solutions as potassium hydroxide (KOH), tetra methyl ammonium hydroxide (TMAH), for instance, or dry etching such as reactive ion etching (RIE) method. Aluminum nitride (AlN), or alternatively zinc oxide (ZnO) is generally adopted for material of the piezoelectric thin film and especially AlN is widely used because of the material behavior that facilitates the matching with the semiconductor manufacturing processes. However, there are problems that in either case of using AlN or ZnO, the etching rate of the piezoelectric thin film is low and sufficient etch selectivity of the piezoelectric thin film to the bottom electrode is hard to be achieved, when the piezoelectric thin film is selectively etched to make the bottom electrode bare so that the extraction wiring can be connected to the bottom electrode. When the etch selectivity of the piezoelectric thin film to the bottom electrode is not sufficient, an over-etching for assuring uniformity of the etching depth decreases the film thickness of the bottom electrode in part of an element region or over the whole element region. Then, series resistance in the bottom electrode increases and contact resistance between the extraction wiring and the bottom electrode increases due to a surface roughness and degeneration of the bottom electrode.
As parameters representing the resonance characteristic of the FBAR, electromechanical coupling factor kt2, which is an indicator of the effectiveness with which a piezoelectric material converts electrical energy into mechanical energy, or converts mechanical energy into electrical energy and Q-value, which is a measure of the sharpness of the resonance peak in the frequency response of the system, are employed. In addition, with regard to the Q-value, there are two Q-values; one is a Q-value at a resonance frequency in which electrical impedance becomes a minimum and another is a Q-value at an anti-resonance frequency in which the electrical impedance becoming a maximum. When a filter is implemented by a combination of resonators, a frequency bandwidth of the filter is proportional to the electromechanical coupling factor kt2, and insertion losses in the frequency band are inversely proportional to a quality factor defined by the product of the Q-value and the electromechanical coupling factor kt2. Since the electromechanical coupling factor kt2 is a value proper to material, there is no necessity for increasing the electromechanical coupling factor kt2, if an appropriate frequency bandwidth can be realized by improving the crystal purity and controlling the crystal orientation to a polarization direction. Therefore, the Q-value must be set as high as possible so as to decrease the insertion losses. Especially, for fabricating the FBAR, not only a material for the piezoelectric thin film but materials for top and bottom electrodes of the FBAR must be considered in the selection of materials. In a piezoelectric resonator implemented by a ceramic material having a film thickness of above 100 micrometers, mass and resistance in the top and bottom electrodes do not affect the resonance characteristic of the piezoelectric resonator, while in a FBAR having a film thickness with only several micrometers, the film thickness of the top and bottom electrodes becomes relatively larger compared with the film thickness of the piezoelectric layer, the resonance characteristic of the FBAR is greatly affected by material properties of the top and bottom electrodes.
Molybdenum (Mo), tungsten (W), iridium (Ir), are regarded to be advantageous as electrode materials of the FBAR, as mentioned in U.S. Pat. No. 5,587,620. However, melting points of such refractory metals are extremely high and have the following problems when a metallic thin film is formed by such refractory metals;
(a) The metallic thin film formed of the refractory metals, is susceptible to residual stress.
(b) A rate of increase in resistivity for the metallic thin film is larger than the increase in resistivity for a bulk metallic material, because the grain size in the metallic thin film is smaller than the grain size in the bulk metallic material.